1. Field of the Invention
The present invention relates to an exhaust gas purification system for an internal combustion engine which has a relatively high thermal durability and can decompose nitrogen oxides (NOx) included in exhaust gas from an engine to purify the exhaust gas.
2. Description of the Prior Art
Japanese Patent Publications No. HEI 1-130735 and No. HEI 1-135541 disclose a Cu-zeolite catalyst capable of reducing NOx in the presence of hydrocarbons (HC) under oxidizing gas conditions.
However, the Cu-zeolite catalyst has a drawback that the Cu-zeolite catalyst easily suffers a thermal degradation and lacks thermal durability. In addition, the Cu-zeolite catalyst needs a considerably large amount of HC to reduce NOx, and therefore requires a special HC supply device.
The research conducted by the inventors confirmed that a noble metal (for example, Pt) base catalyst (or a noble metal base series catalyst), which is known to have a relatively high thermal durability, can reduce NOx at relatively low temperatures and at lean air-fuel ratios.
Though the use of the noble metal base catalyst solves the thermal durability problem and the cost problem of a special HC supply device, it has been found by the inventors that intermediate products such as N.sub.2 O and NO.sub.2 are produced from a portion of the NOx included in the exhaust gas when the noble metal base catalyst is used at relatively low temperatures. Though N.sub.2 O is harmless to a human body, N.sub.2 O adds to the problem of global warming. Therefore, it is desirable to decompose N.sub.2 O into nitrogen (N.sub.2) and oxygen (O.sub.2) as perfectly as possible.